The present invention relates to a lockable cover for a fuel tank. 
Certain plastic fuel tanks have a plastic fill tube. Metal funnels are crimped on to the end of the fill tubes for receiving lockable fuel caps. However, different fuel tanks require different funnels, and one lockable fuel cap may not work with the funnel of a different fuel tank. It is desired to have a lockable fuel tank cover which can be field installed and which can function with a variety of different fuel tanks and funnels. 
A cap locking mechanism was known for use on John Deere 7000 tractors. To use this locking mechanism, the fuel cap was removed, a ring with a long lever arm was mounted over the fill neck, and the fuel cap was replaced. Then a clamping ring was mounted on the fuel cap and locked to the locking ring. The long lever arm would engage the fuel tank and prevent rotation and removal of the fuel cap. However, this previous design is prone to moving around and making noise in response to vibration of the tractor. 